Pulsating combustion is a phenomenon which is well known in the art of air and water heating and is quite extensively discussed in the literature. Examples of patents that have issued in this field are U.S. Pat. Nos. 2,916,032; 3,267,985; 4,241,720; 4,241,723 and 4,309,977 (all to Kitchen). Proposals have also been made to use pulse combustors for material processing. For example, U.S. Pat. No. 4,706,390 (Kitchen) discloses a pulse combustion dehydrator. Other examples of pulse combustion dehydrators are shown in Lockwood's U.S. Pat. Nos. 3,462,955 and 3,618,655. A method and apparatus for conducting a process in a pulsating environment is disclosed in Zinn et al.--U.S. Pat. No. 4,699,588.
Early examples of prior art pulse combustion apparatus are shown in U.S. Pat. Nos. 2,898,978 (Kitchen) and 3,005,485 (Salgo and Kitchen).
Pulsate combustors employed for material processing such a dehydration are unlike those used for heating in that they are required to operate with a strong combustion cycle. This strong cycle is necessary for the production of intense pressure waves, which accelerate the material processing. For example, increasing the rate of drying by imposing sound waves on a material to be dried in the presence of heated air is discussed in "Sonic Drying" by Robert S. Soloff (The Journal of the Acoustical Society of America--Volume 36, No. 5, May 1964).
U.S. Pat. No. 4,697,358 (Kitchen) discloses a pulsating combustor in which a specially designed gas distributor is located downstream of a flame trap. When the combustor is in operation, gas flows virtually continuously into the combustion chamber from a high pressure gas supply. It was found in practice that this design resulted in improved starting of the combustor and a stronger cycle as compared with prior art designs. This combustor was found well suited to use in a pulse combustion dehydrator.
With this background, an object of the present invention is to provide improvements in pulse combustion apparatus primarily (but not exclusively) for use in material processing.
The apparatus provided by the invention includes a combustion chamber having inlet means for fuel charges and an outlet for exhaust gases remote from the inlet means. An exhaust pipe (sometimes called a "tail pipe") extends from the exhaust gas outlet and forms a reasonant system with the combustion chamber. The apparatus also includes means operable to initiate combustion in the chamber. In accordance with the invention, the apparatus further includes low pressure fuel gas supply means including a gas cushion chamber having an inlet provided with a metering orifice and an outlet. The fuel charge inlet means of the apparatus includes a fuel gas distributor disposed within the combustion chamber and defining an annular combustion air passageway around the distributor. The distributor has a fuel inlet connected to the outlet of the gas cushion chamber and a plurality of discrete fuel gas outlets from which individual gas streams issue into the combustion chamber when the apparatus is in operation. Pressure responsive valve means is provided for controlling communication between the fuel inlet and the gas outlets so and is arranged to interrupt the flow of gas to the outlets during high pressure portions of the pulse combustion cycle. Pressure responsive valve means is also provided for admitting air to the annular combustion air passageway during low pressure portions of the pulse combustion cycle. An annular flame trap extends across the combustion air passageway upstream of the fuel gas outlets of the distributor and the flame trap is adapted to provide a relatively smooth unturbulated flow of air into the combustion chamber.
The invention is based on the discovery that some increase in the amplitude of the combustion cycle (and hence a stronger cycle) will result from the use of low pressure gas with valving to stop the gas during high pressure portions of the pulse combustion cycle. This action is believed to reduce the amount of residual flame in the combustion chamber which persists during low pressure portions of the cycle. The use of an annular flame trap providing a relatively smooth, unturbulated flow of air also helps to reduce the residual flame. It is believed that residual flame is necessary for ignition of successive fuel charges after initial starting of the pulse combustion process but that residual flame should be kept to a minimum when a strong cycle is required.
Preferably, the exhaust pipe of the apparatus includes an inner portion adjacent the combustion chamber of a first diameter and an outer portion adjacent said inner portion of increased diameter. A fluid inlet is then provided in the outer portion of the exhaust pipe at a location adjacent the inner portion for permitting a fluid to be drawn into the pipe by pressure waves returning to the combustion chamber during low pressure portions of the pulse combustion cycle and venturi action due to the increase in diameter of the exhaust pipe. In a material processing application, a fluid appropriate to the particular process may be permitted to enter the exhaust pipe through the fluid inlet. Alternatively, the fluid may be air. Preferably, the fluid inlet is provided with a pipe through which the fluid enters the exhaust pipe.
In an apparatus which includes a cushion chamber for combustion air communicating with the combustion chamber inlet means, the pipe may be an air bypass pipe extending between the air cushion chamber and the fluid inlet in the exhaust pipe. As compared to an open pipe, this arrangement has the advantage that the air cushion chamber provides a noise muffling effect.
It has been found that the combustion cycle is strengthened by the use of an air bypass pipe and that the addition of low temperature (higher density) air to the exhaust at the location referred to above apparently increases the compression in the combustion chamber produced by the returning pressure waves. In a material processing application, material can be introduced into the exhaust pipe through the bypass pipe since there is considerable suction in the pipe. In other words, the bypass pipe can also serve as a material inlet. This may be additional to or instead of a special material inlet at the same location in the exhaust pipe.
The invention also provides a dehydrator including a pulse combustor having a combustion chamber and an elongate exhaust pipe forming a reasonant system with the combustion chamber, the exhaust pipe having an inlet for material to be dehydrated and an outlet through which the material is discharged in a pulsating exhaust gas stream when the combustor is in operation. The dehydrator also includes a motor driven rotary drum having a first end into which the exhaust pipe of the pulse combustor extends, and a closed second end. The drum extends about a rotary axis and is positioned with its said axis inclined upwardly towards the said closed end so that material introduced into the drum from the exhaust pipe is directed towards said closed end but tends to return towards the first end of the drum under the effect of gravity. The drum is provided internally with mechanical means for assisting return of the material towards the first end of the drum and the drum has a material outlet at its first end. The dehydrator further includes a cyclone separator having a cyclone chamber provided with an inlet for receiving a gas stream containing suspended material and in which said stream and material are separated. A dried material outlet is provided at a lower end of the chamber and the separator also has a gas stream outlet. The material inlet of the cyclone chamber is positioned above the material outlet of the rotary drum and the two are connected by duct means extending upwardly from the material outlet to the cyclone chamber inlet.